Elbow prostheses are generally totally restrained (hinged), a semi-restrained, or unrestrained (hingeless). The amount of damage to the natural joint which is desired to be replaced determines the type of elbow prosthesis to be implanted in place of the diseased or damaged natural joint. Unrestrained (hingeless) prostheses are available which provide good joint stability with minimum bone removal while providing normal flexion and extension motion as there is no built-in constraint producing strain on the seating of the prosthesis. However, such minimally constrained surface replacement elbows can only be used when the bone shaft is still strong and its soft tissue/ligaments are functional as these prostheses rely on the elbow's ligamentous structure to maintain joint stability.
Unfortunately, most elbows that require replacement have ligamentous laxity or instability and/or inadequate bone stock which consequently requires use of the "constrained type" of elbow prosthesis. Orthopaedic devices of the constrained or semi-constrained type employ two arms or spikes which are inserted in the medullary canals of the humerus and the ulna, and are rotatably interconnected via a hinge to provide stability. While total elbow prostheses are known which have a semi-constrained interface at the humeralulnar joint, the arms or spikes of these devices very often cause erosion of the humerus and ulna bones due to the inability of the prostheses to dissipate significant joint stresses. For example, when an elbow is subjected to forceful extension, such as may occur when throwing a baseball, presently available prostheses do not provide the damping or shock absorbing action usually provided by the natural soft tissues of the elbow.
Furthermore, the hinged joint of known prostheses actually enables the shock to travel through both spikes of the prosthetic device, placing stress on the bones in which the spikes are inserted. The jolts and forces associated with this stress gradually chip away at the bone and, after a period of time, the spikes' adhesive contact with the medullary canal of the bones is loosened. Consequently, after a year or two, replacement of these prostheses is necessary. Moreover, the deterioration of the bone canal or shaft may make subsequent replacements of the joint more difficult or impossible.
Orthopaedic devices for use in total elbow replacement which address the need for normal flexion and extension motion and joint stability are represented by U.S. Pat. Nos. 3,816,854; 4,079,469; 4,224,695; and 4,383,337. All of these prostheses employ the single pivot hinge arrangement as discussed above.
U.S. Pat. No. 4,777,941 to Borig et al. discloses an orthopaedic hinge for limiting the flexion and extension of an associated body part, for example, a knee or elbow. The hinge taught by Borig et al. employs a housing and a pair of control arms which are rotatably mounted about respective pivot points in the housing. However, this bi-pivotal hinge is for use in a brace which externally supports the associated joint which may be an orthopaedic implant. The bi-pivotal arrangement minimizes bending moments created at the joint due to misalignment of the centers of the external hinge and the joint. Nowhere does Borig et al. suggest that its external bi-pivotal hinge can be used in an implant for replacing a joint or that its hinge reduces stresses which travel through the arms of an orthopaedic hinge.
In so far as applicant is aware, all prostheses for elbow replacement employ a single pivot hinge resulting in an undesirable connection between the spikes of the orthopaedic device. This humeralulnar joint connection enables stress to travel through both spikes, eventually damaging the humerus and ulna bones. According to conventional practices, constrained prostheses should have little play in order to compensate for the instability of the natural ligaments. The additional joint stability, however, increases the amount of stress which travels through the spikes and causes the deterioration of the natural bones as described above.
Thus, it can be seen that there is a need for an orthopaedic device suitable for a total elbow replacement which provides the necessary constraint for joint stability but which has spikes which are not interconnected and thus tightly affixed together. Accordingly, the transfer of strain or deformation to the medullary canals of the bones is prevented, as is the resultant deterioration of the humerus and ulna.